The present invention relates to an electrochromic display device, and particularly to such a display device having an organic insulating layer for increasing the lifetime of the transparent electrodes.
Electrochromic devices are well-known devices which exhibit a phenomenon known as "persistent electrochromism", e.g., see U.S. Pat. No. 3,521,941 entitled, "Electro-Optical Device Having Variable Optical Density", issued July 28, 1970. The term "persistent electrochromism" denotes the property of a material whereby its electromagnetic radiation absorption characteristic can be altered, in most instances, even at ambient temperature. In order to alter the absorption characteristic, an electric field is provided such that an electric current flows through the electrochromic material. Such materials, for example, may exhibit little or no absorption of visible wavelength in the unswitched state, and therefore be transparent, but when subjected to an electric switching current, effectively absorb in the red end of the spectrum, turning blue in color. Similar effects can be observed in other portions of the electromagnetic spectrum, invisible as well as visible.
The phenomenon of "persistent electrochromism" has also been exhibited in electrochromic devices which include an electrolyte-electrochromic sandwich wherein the electrolyte functions both as a conductive medium and as a source of positive ions. For example, the sulfuric acid electrolyte of U.S. Pat. No. 3,708,220, issued Jan. 2, 1973. In these devices, the electrolyte is chosen to be sufficiently conductive so as to permit low voltage operation of the electrochromic device while also being chemically compatible with the electrochromic layer and electrode employed in the device.
Such display devices have been developed and may be successful for many applications, e.g., alpha-numeric displays. In the construction of an electrochromic alpha-numeric cell, it is usually necessary to etch patterns in the transparent front electrode. When the complete cell is assembled, there are regions where the electrolyte is sandwiched between the transparent electrode and a counter electrode. In these regions, wasteful leakage current flows when a voltage is applied. If inorganic acids, such as sulfuric or phosphoric acid, are used to increase the conductivity of the electrolyte, then the leakage current will undesirably cause the transparent electrodes to be rapidly etched away. Thus, devices which employ the electrolyte have been found to operate well only for a rather limited number of cycles i.e., several hundred thousand, as the direct contact of the electrolyte with the transparent electrodes appears to cause the transparent electrodes to deteriorate. The deterioration causes lack of complete switching in the display and eventually, failure. Thus, it would be desirable to develop an electrochromic display device in which the transparent electrodes exhibit an increased lifetime, i.e., continued operation without any significant degradation.